The invention relates to a self-contained ride level control system for a motor vehicle with which the current ride level of the motor vehicle can be raised and lowered and which contains the following components:                a compressor,        a compressed air accumulator which can be filled with air from the atmosphere and which can be emptied into the atmosphere,        at least one air spring, the air spring being connected to the compressed air accumulator via the compressor in such a way that compressed air can be transferred into the compressed air accumulator from the air spring and in the opposite direction.        
Such a self-contained ride level control system is known, for example, from DE 101 22 567 C1. In the ride level control system which is known from this publication, the accumulator pressure of the compressed air accumulator is controlled indirectly by determining the quantity of air in the ride level control system by filling the compressed air accumulator with air from the atmosphere when the quantity of air in the ride level control system is below a lower limit, and the compressed air accumulator is emptied into the atmosphere when the quantity of air within the ride level control system is above an upper limit. Here, the control is performed in such a way that the quantity of air in the ride level control system is in a working range between the lower and upper limits after the filling or emptying process. The method therefore ensures that the quantity of air in the ride level control system is always kept in a certain working range and as a result the accumulator pressure in a specific state of the motor vehicle is also always in a specific range. The working range for the quantity of air is selected here in such a way that in a specific normal state of the motor vehicle the accumulator pressure is in a range in which both rapid raising and lowering of the motor vehicle are possible. The accumulator pressure merely needs to be equalized by filling or emptying the compressed air accumulator if, owing to leakage or large temperature fluctuations, it has become apparent that the quantity of air in the ride level control system is outside the working range (and consequently the accumulator pressure is outside the desired range). The compressor of the ride level control system is therefore activated only rarely in order to fill the compressed air accumulator so that the compressor running times are shortened and the compressor service life is extended. However, it is to be noted that the compressed air accumulator of the ride level control system is filled or emptied even if the quantity of air in the ride level control system is outside the working range even though in this case it would have still been possible to perform a large number of requested ride level changes for the motor vehicle using the ride level control system. This case can occur, for example, if the motor vehicle has been parked in a warm garage and is subsequently driven in a cold environment. Owing to the drop in temperature the quantity of air in the ride level control system is then reduced and as a result the compressed air accumulator is filled if the quantity of air is outside the working range. The filling process is performed even though a large number of requested control processes would still be possible with the ride level control system.
The invention is based on the object of providing a self-contained ride level control system for a motor vehicle in which the accumulator pressure has to be adjusted as rarely as possible by filling or emptying.